1. Field of the Invention
The present invention relates to a process for producing an alkali metal hydroxide. More particularly, it relates to a process for producing an alkali metal hydroxide and chlorine by electrolyzing an aqueous solution of an alkali metal chloride at a low voltage by using a cation exchange membrane of a fluorinated polymer having carboxylic acid groups.
2. Description of the Prior Art
It has been proposed to use an ion exchange membrane in place of asbestos as a diaphragm to produce an alkali metal hydroxide by electrolyzing an aqueous solution of an alkali metal chloride so as to obtain an alkali metal hydroxide having high purity and high concentration.
In view of an alkali resistance and a chlorine resistance, a cation exchange membrane of a fluorinated polymer is preferably used as such ion exchange membrane and carboxylic acid groups are preferably selected as such cation exchange groups since an alkali metal hydroxide is produced at a high concentration in a high current efficiency.
On the other hand, recently energy saving has been considered in the world. In view of the energy saving, it is preferable to minimize a cell voltage for such technology.
Various manners for minimizing the cell voltage such as improvements of the materials, compositions and configurations of the anode and the cathode and selections of the composition of the cation exchange membrane and the kind of the ion exchange groups have been proposed. In accordance with such manner, certain effects have been found. However, the maximum concentrations of the resulting alkali metal hydroxide have not been so high in many cases. If the concentration of the alkali metal hydroxide is over the maximum level, it has not been satisfactory as an industrial process because of disadvantages of a severe increase of the cell voltage, a decrease of the current efficiency and the deterioration of membrane and durability of the cell voltage lowering phenomenon.
It has been proposed to incorporate a compound having an anion such as a phosphate which forms a water insoluble gel with a polyvalent cation such as calcium ion remained as the impurity in the anolyte as the solution of an electrolyte in a process for producing an alkali metal hydroxide by using an ion exchange membrane (U.S. Pat. No. 3,793,163). The polyvalent cation contained in an aqueous solution of an alkali metal chloride is penetrated into the ion exchange membrane to reduce a mobility of an alkali metal ion or to cause cracks in the membrane in an electrolysis by using a cation exchange membrane having sulfonic acid groups. The above-mentioned proposal is to prevent the penetration of the polyvalent cation into the cation exchange membrane whereby the characteristics of the membrane are maintained and the decrease of the current efficiency and the increase of the cell voltage are prevented.
In view of these considerations, cation exchange membranes of a fluorinated polymer having carboxylic acid groups as the ion exchange groups which are considered to give advantageous effects of high concentration of the resulting alkali metal hydroxide and current efficiency have been further studied. It has been found the following different phenomenon by using a cation exchange membrane of a fluorinated polymer having carboxylic acid groups at a specific content of the ion exchange groups instead of the cation exchange membrane having sulfonic acid groups. When a polyvalent cation preferably a specific cation which is considered to cause the trouble is incorporated or added without adding an anion such as phosphoric acid ion for forming the compound, the cell voltage can be reduced without any deterioration of the current efficiency as a surprising phenomenon.